Vehicle transmissions are generally classed as manual or automatic. In a manual transmission, the vehicle driver determines the speed and smoothness of a ratio change, principally by careful control of a disengageable clutch.
In an ‘automatic’ transmission, such ratio changes are determined by a control system, in which the rate of de-clutching and re-clutching is not determined by the vehicle driver. Such transmissions may generally be operated in fully automatic mode, or permit a driver to initiate a ratio change on demand. Automated manual transmissions, that is to say manual transmissions having actuators to move a shift lever and clutch, fall into the automatic class for the purpose of the present invention by virtue of the provision of a control system for de-clutching and re-clutching drive.
It has long been realised that management of energy during a ratio shift is somewhat problematic. In a manual transmission a vehicle driver manages energy transmitted through the clutch by slipping the clutch just sufficiently to ensure smooth disengagement and re-engagement of drive. Generally speaking the driver will make a ratio change as quickly as possible, whilst avoiding an abrupt take-up of drive, which could be uncomfortable for vehicle occupants. A prolonged take-up of drive should generally be avoided so as to obviate excessive wear of the clutch. A driver must also deal with variations of load and gradient, and avoid an excessive increase in engine speed if passing through a neutral.
Ensuring appropriate shift energy during an automatic ratio change is not an easy task because of the wide variations of vehicle use, and also because clutch performance may vary due to age and other external factors, such as temperature. In particular, engine torque must generally be modulated to meet the instant demand of the transmission, so that for example a newly selected speed ratio is neither engaged too slowly nor too quickly. A variation in the speed of clutch engagement may be required depending upon, for example, accelerator pedal position.
Systems and methods of determining the engine torque requirement during a ratio change are known, and form no part of the present invention. Engine torque requirement can for example be contained in a look-up table of an engine electronic control unit (ECU), or determined by reference to an appropriate algorithm, having regard to conventional factors such as engine speed, road speed, and driver demand. Generally speaking a request for a change in torque demand should be implemented at the engine as rapidly as possible, and substantially instantly.
In a gasoline engine, rapid response to demands for torque change is affected by the volume of air in the inlet manifold, downstream of the usual throttle valve and upstream of the engine inlet valve(s). Responding to a demand for torque change by changing the position of the throttle valve may be characterized as ‘slow’ since the air already in the inlet manifold will affect engine power output for the next few combustion events. Eventually the volume of inlet air to the cylinder(s) will change as the throttle valve is adjusted, so that the torque output matches demand; however this response rate is not sufficiently fast to meet requirements during a change of speed ratio in a transmission.
One combustion factor which can be quickly changed is the timing of an ignition spark at the sparking plug. The speed of response may be at least an order of magnitude faster than the effect of changing throttle valve position, and may be effected within one TDC (top dead centre) of the engine.
In order to ensure a fast response to a demand for torque reduction during a ratio change, it is known to change ignition timing to reduce the power produced during a combustion event, in anticipation that an increase in power will be required as the new ratio is engaged. The increase in power can be quickly achieved by changing ignition timing to the optimal position for efficient combustion, without waiting for the volume of air to be increased. A fast response of this kind can be implemented cylinder by cylinder, so that successive firing events of a multi-cylinder engine may have different timing of the ignition spark.
Thus, by way of example, a multi-cylinder engine may be always assumed to be subject to an imminent torque-down demand followed by a torque-up demand to the original level, during a speed ratio upshift. Fuelling is generally commensurate with air volume in order to achieve stoichiometric combustion.
The torque-down demand is required, in order to avoid driveline shock or shunt, and is implemented by a retardation of the timing of the ignition spark; a torque-up demand is implemented by a re-advancement of the timing of the ignition spark. This conventional solution, whilst effective, has the effect of reducing combustion efficiency during the period of deliberate ignition retardation, with the consequence of a reduced torque output. However, inefficient combustion results in additional waste heat to be absorbed by the engine cooling system, increased fuel consumption, and unnecessary noxious exhaust emissions. This problem is exacerbated in current multi-speed transmissions, which may have as many as nine to ten forward speed ratios, with a consequent increase in the number of up and down changes which are required during normal driving, as compared with a conventional four or five speed transmission.
What is required is a means of providing rapid response to a demand for a reduction or an increase in engine torque, but which does not rely upon the inefficient combustion method noted above.